Cellulose is an unbranched polymer of glucose linked by β(1→4)-glycosidic bonds. Cellulose chains can interact with each other via hydrogen bonding to form a crystalline solid of high mechanical strength and chemical stability. The cellulose chains are depolymerized into glucose and short oligosaccharides before organisms, such as the fermenting microbes used in ethanol production, can use them as metabolic fuel. Cellulase enzymes catalyze the hydrolysis of the cellulose (hydrolysis of β-1,4-D-glucan linkages) in the biomass into products such as glucose, cellobiose, and other cellooligosaccharides. Cellulase is a generic term denoting a multienzyme mixture comprising exo-acting cellobiohydrolases (CBHs), endoglucanases (EGs) and β-glucosidases (BGs) that can be produced by a number of plants and microorganisms. Enzymes in the cellulase of Trichoderma reesei include CBH I (more generally, Cel7A), CBH2 (Cel6A), EG1 (Cel7B), EG2 (Cel5), EG3 (Cel12), EG4 (Cel61A), EG5 (Cel45A), EG6 (Cel74A), Cip1, Cip2, β-glucosidases (including, e.g., Cel3A), acetyl xylan esterase, β-mannanase, and swollenin.
Cellulase enzymes work synergistically to hydrolyze cellulose to glucose. CBH I and CBH II act on opposing ends of cellulose chains (Barr et al., 1996, Biochemistry 35:586-92), while the endoglucanases act at internal locations in the cellulose. The primary product of these enzymes is cellobiose, which is further hydrolyzed to glucose by one or more β-glucosidases.
The cellobiohydrolases are subject to inhibition by their direct product, cellobiose, which results in a slowing down of saccharification reactions as product accumulates. There is a need for new and improved cellobiohyrolases with improved productivity that maintain their reaction rates during the course of a saccharification reaction, for use in the conversion of cellulose into fermentable sugars and for related fields of cellulosic material processing such as pulp and paper, textiles and animal feeds.